Various four wheel vehicle steering systems have been proposed for the dual purpose of improving the vehicle stability and maneuverability during turning maneuvers, and improving the on-center responsiveness, as compared to a two wheel steering system. The stability improvement, achieved by steering both rear wheels at a relatively small angle in-phase with the front wheels, is implemented primarily at moderate-to-high vehicle speeds. The on-center response improvement, achieved by steering both rear wheels at a relatively small angle out-of-phase with the front wheels, is also implemented primarily at moderate-to-high vehicle speeds. The maneuverability improvement, achieved by steering both rear wheels at a relatively large angle out-of-phase with the front wheels, is implemented primarily at low vehicle speeds.
Since the above-described controls steer both rear wheels in unison, the rear wheels each experience toe-in and toe-out steering movements. The toe angle is defined as the steering angle of the wheel relative to the longitudinal or roll axis of the vehicle. Toe-in refers to a toe angle which points the wheel toward the forward longitudinal axis of the vehicle, and toe-out refers to a toe angle which points the wheel away from the forward longitudinal axis.
The above-described characteristic of conventional rear wheel steering systems causes several problems. Perhaps most importantly, the vehicle must be designed to accommodate relatively large peak-to-peak movement of the rear wheels. This, of course, impacts styling, tire and wheel selection, trunk capacity, etc. Furthermore, special toe angle alignment procedures and equipment must be used. Also, failure analysis and detection becomes very complicated, especially if electrical actuators are employed.